Embossing techniques for producing integrated optical circuits

ABSTRACT

There is disclosed a technique for producing integrated optical waveguides or circuits in which a substrate is embossed by a die of the desired guide pattern and is then coated with a liquid that fills the grooves and has sufficient surface tension to yield a greater thickness in the grooves than over adjacent substrate material. The liquid may be a solution of a higherindex optical material, solidifying in the grooves during evaporation of the solvent. Alternatively, a liquid monomer of a polymerizable higher-index material may be used which is subsequently polymerized in the grooves. This embossing and filling technique is particularly economical when a large number of replicas of a given optical circuit are to be produced.

United S1 Chandross et al.

EMBOSSING TECHNIQUES FOR PRODUCING INTEGRATED OPTICAL CIRCUITS Assignee:Bell Telephone Laboratories Incorporated, Murray Hill, NJ.

Filed: Oct. 14, 1971 Appl. No.: 189,101

SUBSTTTUTE FOR MIQQING x12 EARQH RQQM.

2,892,716 6/1959 Martin ll7/l38.8 A

3,084,068 4/1963 Munn ll7/l6l UB 3,278,322 10/1966 Harkins et a1. 117/103,671,283 6/1972 Crowley et al 117/10 Primary Examiner-William D. MartinAssistant Examiner-Theodore G. Davis Att0rneyW. L. Keefauver [57]ABSTRACT There is disclosed a technique for producing integrated opticalwaveguides or circuits in which a substrate is embossed by a die of thedesired guide pattern and is then coated with a liquid that fills thegrooves and has sufficient surface tension to yield a [52] Cl 1117/8,7/9331 7/1383 A greater thickness in the grooves than over adjacentll7ll6| substrate material. The liquid may be a solution of a 511 Int.Cl. B44c 1/22 higher-index I material, solidifying the 58 Field ofSearch 117/8, 161 UB 11 durmg evaporatim 0f the wivem- Alterna- 117/l338 A 333 9331. (n/D163 tively, a liquid monomer of a polymerizablehigherindex material may be used which is subsequently po- [56]References Cited lymerized in the grooves. This embossing and fillingtechnique is particularly economical when a large UNITED STATES PATENTSnumber of replicas of a given optical circuit are to be 2,161,990 6/1939Asnes 161/5 pm'ducm 2,320,536 6/1943 Pollack et al.... ll7/16l UB2,361,055 10/1944 Pollack 117/161 UB 7 Claims, 8 Drawing FiguresMATCHING GUIDE INTERFACE T RAN SPARE NT P LA ST 1 C COAT SUBSTRATE TOFILL GROOVES PATENIEDO0I23 1915 3.7673145 SHEET 10F 2 F/G. MAKE A DIEMATCHING GUIDE INTERFACE EMBOSS TRANSPARENT PLASTIC SUBSTRATE COATSUBSTRATE TO FILL GROOVES F/GZA GLASS II F/G. 3/4 GLASS Il SOLUTION DEPOSITED FILM OF FIG. 20 HIGH INDEX F/G. 30 HIGH 'NDEX MATERI POLYMER W 7AL q 26 PATENIIIBnmza ma SHEET 2 OF 2 FIG. 4

MAKE A FIRST DIE FOR A CIRCUIT TO BE REPLICATED MAKE A MASTER DIE FROMTHE MOLD EMBOSS SUBSTRATE WITH MASTER DIE COAT SUBSTRATE WITH CIRCUITMATERIAL EMBOSSING TECHNIQUES FOR PRODUCING INTEGRATED OPTICAL CIRCUITSBACKGROUND OF THE INVENTION This invention relates to techniques formaking optical waveguiding devices and related optical circuits.

Recently there has been considerable interest in thin film optical'circuits and other dielectric waveguiding circuits. Several types oflight guiding arrangements and input and output coupling arrangementshave been investigated.

Nevertheless, most of the experiments to date have merely indicated theneed for further research and development because each of the possibleconfigurations has some drawback. For example, the thin film opticalguides use thin films that are so wide that they guide the light in onlyone dimension. For many applications in integrated optical circuits, thelight must also be guided in the transverse direction; and guides withcross sections of about three micrometers wide by about one micrometerthick are typically desired in order to obtain single-mode operation. Asone example for the poten- I tial application of such integrated opticalcircuits, it is I noted here that the lossesoccurring in long-distanceoptical fiber transmission lines require amplifying and pulse-shapingrepeaters at intervals of typically every 10 km. Such repeaters are,therefore, required in large numbers and the problem reduces to one ofsimple and economical circuit fabrication, particularly the economicalfabrication of replicas of a given circuit.

SUMMARY OF THE INVENTION Our invention is based on our recognition thatoptical waveguide circuits can be produced by a replicating method thefirst step of which is reminiscent of, but different from, the massproduction of phonograph records.

According to our invention, a transparent, plastic substrate is embossedby a die of the desired circuit pattern. In the second step offabrication, the substrate is then coated with a higher-indextransparent dielectric material that adheres to the substrate and fillsthe impressions, typically grooves, made by the die. This material isapplied in liquid form so that it has sufficient surface tension toyield a greater thickness in the grooves than on adjacent substratematerial. The region of greater thickness is then an optical guide orother part of an optical circuit.

In a preferred embodiment of our invention, the higher-index transparentdielectric material is chosen to be the liquid monomer of apolymerizable material which is then polymerized in the grooves, thepolymerization being initiated either photochemically or thermally.Other types of initiation such as exposure to electrons or high energyradiation are also feasible. The term plastic" in the foregoing summaryis used in its generic sense, referring to the permanent, nonreturningdeformability of a first material by a die of a second harder materialwithout fracture or any qualitative internal structural change. Thedeforming is done typically under high pressure and at elevatedtemperature.

BRIEF DESCRIPTION OF THE DRAWING Further features and advantages of ourinvention will become apparent from the following detailed description,taken together with the drawing, in which:

FIG. 1 is a block diagrammatic flow chart of the successive steps of themethod of our invention;

FIGS. 2A through 2C show structure that are related to the performanceof the steps of the flow chart;

FIGS. 3A through 3C show structures like those of FIGS. 2A through 2Cexcept for a materials modification yielding a modified final product;and

FIG. 4 is a modified block diagrammatic flow char of the successivesteps of a specific method according to our invention.

DESCRIPTION OF THE ILLUSTRATIVE EMBODIMENT The method described in theblock diagrammatic flow chart of FIG. 1 is a new technique forfabricating integrateigpticaLwaygggidegrcuits; and this method has thepotential for simple and inexpensive mass production. The sequence ofsteps will perhaps be better understood if certain aspects of the secondstep, the embossing step, are discussed in terms of our earlyexperiments.

We have found that it is possible to emboss a rfla stic' su bstrat e,such as poly(methylmethacrylate), PMMA, to produce a pattern ofimpressions, typically grooves. These grooves then form part of theguiding interfaces for the optical waveguide components that aresubsequently formed by filling the grooves. It is this indented orgrooved surface of the embossed substrate which we refer to hereafter asthe guide interface.

The plastic substrate can be any material that can be deformed withoutqualitative internal structural change or fracture by a suitably harddie. The material must be sufficiently transparent and homogeneous forthe intended purpose, and is preferably optically isotropic.

Obviously, then, the first step in the fabrication of an optical circuitis to make a die having a surface matching the desired guide interface,for example, matching the groove surfaces referred to above. As anillustration, this step can be implemented by a structure of the typeshown in FIGS. 2A or 3A. Specifically, a glass plate 11 has a quartzfiber 13 typically of diameter B 5 micrometers cemented to it by meansof an epoxy compound 12. While the fiber 13 is shown in cross section,it should be understood that it extends backwardalong the line of sightand is cemented to the glass plate throughout its length. It can also bebent or curved within a surface parallel to the surface of the glassplate if it is desired to make a curved guiding structure in thesubstrate. Moreover, it is not necessary that the fiber has a roundcross section. Elliptical, triangular or rectangularcross sections areequally acceptable from an optical point of view, although the embossingwill work more accurately with the smoother profiles.

Dies for the embossing step could also be produced by optically writingthe original circuit pattern in a film of positive working photoresist,which is then used as a mold in forming the die pattern in metal. Thistechnique is analogous to that which has been demonstrated for theproduction of phase holograms in vinyl tape, as described by R.Bartolini et al. in their article in Applied Optics, Vol. 9, page 2283(1970). Alternatively, the die could be made as suggested in the firstthree steps of FIG. 4, described below.

The second step of the fabrication sequence, FIG. 1, the embossing step,can be implemented by preparing a substrate 14 to have a top surfaceessentially parallel to the original bottom surface of glass plate 11.The

substrate 14 is illustratively a sheet of poly(methylmethacrylate), awell-known plastic material having a refractive index of 1.49. The dieis positioned with plate 11 parallel to substrate 14; and quartz fiber13 is then pressed at a temperature of typically 100 degrees centigradeagainst substrate 14 with a pressure sufficient to form the groove 15shown in cross section but which extends back along the line of sight.Groove 15 can have any bends or curves initially provided in theconfiguration of fiber 13. The die and substrate are allowed to cooldown while still under pressure, and then they are separated. Theresulting groove 15 is the guide interface. Its depth, W in FIGS. 28 and3B, is typically 0.5 to micrometers.

To form an optical guide in the groove 15, the last,

step of he sequence of FIG. 1, the filling, is carried out. This step isequally as important to our fabrication technique as is the embossingstep. Ideally, to give the strongest light-guiding effect, i.e., toreduce loss of light in curves of the guide, only the groove or groovesshould be filled; and there should no film on. the areas of thesubstrate directly adjacent to the groove. In practice, however, a verythin continuous film of the higherindex material is tolerable in theseareas, provided its thickness W; (see FIG. 2C) is below the cutoffthickness for the lowest order guided mode in that film. Typically, W;0.2 micrometer is required. The refractive index of the solidifedfilling material must be higher than that of the substrate, typically byone percent. Furthermore, the filling material should have low opticallosses. Obviously, to be compatible with mass production techniques, thehigh-index filling material must be deposited over the entire surface ofthe substrate 14. To attempt to deposit it in a limited region onlywould make the cost of production unreasonably high. Therefore, it isapparent that, for this step, a liquid high-index material is requiredwhich should be able to wet, or adhere, to the surface of substrate 14.Yet, it must have a sufficiently high surface tension to form a smoothsurface over the groove 15, yielding a greater thickness W of thehigh-index material in the groove than the thickness W; on thesurrounding surface of substrate 14.

For the implementation of the filling step we distinguish between twoalternative methods. In the first method, the filling material is aliquid solution of a high-index optical material. The liquid filmapplied to the substrate solidifies upon evaporation of the solvent.

Because the solid content of suitable solutions is low,

typically not more than 20 percent, the solidification is accompanied bya very considerable volume shrinkage. This results in filled grooveswhose top surface is indented, as in FIG. 2C, although the indentationis shallower than the groove 15. Therefore, the solutiondepositedpolymer film 16 is thicker in the embossed groove between surfaces and17 than at any other point over the substrate material 14. Since theindex of refraction of material 16 is higher than the index ofrefraction of substrate 14, light guiding can occur in the regionbetween surfaces 15 and 17 along the groove.

The other possible implementation of the filling step is to use anundiluted, liquid monomer of a polymerizable material of higherrefractive index. Because the polymerization occurs with littleshrinkage, the solid film resulting from this filling technique has anessentially flat top surface 26, as shown in FIG. 3C. Therefore, if theliquid monomer is allowed to evaporate for some time before thepolymerization to produce a film of the desired thickness W,,, guideswith much larger ratio W,,/W,, i.e., with stronger guiding, can beproduced by this alternative technique.

Below, we will discuss both filling methods in more detail. Thetechnique of depositing light-guiding films from solution has beenstudied recently, as disclosed in the copending patent application oftwo of us, R. Ulrich and H. P. Weber, Ser. No. 131,296, filed Apr. 5,197], and now US. Pat. No. 3,725,809. On glass substrates some of thematerials which they investigated were found to form films with very lowoptical losses (0.3 dB/cm). We have shown that deposition from asolution can also be used for filling embossed grooves.Solution-deposited films are typically applied in the form of about a 10percent solution which then shrinks to the desired thickness as thesolvent evaporates. The thickness of the final film is approximatelyproportional to the initial thickness of the applied liquid film. On anembossed substrate this initial film will be thicker in the grooves thanin the unembossed areas; thus, the final film will also be thicker inthe grooves. However, because of the large shrinkage (about percent)during solvent evaporation, the difference between the thicknesses ofthe final film in the embossed and unembossed regions may not be aslarge as desirable. Careful control of the initial liquid film thicknessis therefore necessary in this technique. In using solution-depositedfilms, the solven used to deposit the film should be carefully selectedto avoid its attacking the substrate, either distorting or dissolvingthe embossed pattern, or producing a rough interface between film andsubstrate.

The alternate technique for filling the grooves, which we feel offersmany advantages, is to lay down a thin film of an undiluted liquidmonomer (e.g.,cyclohexyl methacrylate) and then to photoinitiatepolymerization of the film. In this technique, the film thickness can beadjusted by controlled evaporation of a part of the iquid film beforepolymerization is initiated. The total shrinkage during polymerizationdepends on the specific monomer, but it is typically only about 10percent; thus it should be possible to obtain relatively largerthickness differences between the film in the embossed grooves and thatin the unembossed areas. With this filling technique the thickness ofthe initial liquid film is much less critical than with solutiondeposition. Since we start with a pure material (except for a fewpercent of a photosensitive initiator) there is also no problem ofsolvent effects on the substrate, although, of course, the liquidmonomer must be compatible with the substrate. 7 A wide variety ofacrylic esters may be used as filling material. By choosing appropriatemonomers or combinations of them, it is possible to adjust therefractive index of the film over a considerable range. Other monomerssuch as derivatives of styrene could also be used in an analogousmanner. Furthermore, one could fill the grooves with various kinds ofepoxy resins. To avoid the use of a diluent, the epoxides should be ofrelatively low molecular weight, and consequently low viscosity, andshould be applied at a temperature that causes them to flow into thegrooves.

It is also possible, if desired, to dope the groovefilling material orthe substrate with a suitable dye of the type used for dye lasers inorder to provide, with suitable optical pumping, gain for the lightguided in the waveguide.

For our initial experiments on the preparation of embossed guides wemade dies by cementing 5 micrometer diameter glass fibers 13 to glassslides 11. Specifically, a microscopic slide 11 was coated with a thin(about 3 micrometer thick) layer of epoxy cement (CIBA Aralditecomponents 509 and 591); glass fibers were laid on the cement, and theepoxy was then cured for about 2 hours at 100 degrees Centigrade. FIGS.2A and 3A show the approximate cross section of the resulting die. Forembossing, the die was brought in contact with substrate 14 of PMMA(commercial Plexiglas), with a pressure of about 5 pounds per squareinch. Then it was heated to about 100 degrees centigrade. Upon cooling,the die separated from the plastic; the fiber remained intact andcemented to the die. An approprimate cross section of the resultinggroove in the plastic substrate is shown in FIGS. 2B and 3B. The grooveswere about 3 micrometers wide by about 1 micrometer deep, just aboutideal for optical waveguides. Using this technique, we have been able tomake many impressions from the same die. The embossed grooves have quitesmooth walls, and we feel that carefully controlled annealing could makethen still smoother.

We demonstrated the filling of the grooves by solution deposition usingthe same epoxy as was used to glue the quartz fibers to the die. Amixture of CIBA of substrate 14 was formed between surfaces 15 and 17,as shown in FIG. 2C. Light of 0.633 micrometer wavelength was coupledinto the guide using a prismfilm coupler of known type, as in US. Pat.No. 3,584,230, issued June 8, 1971, and the light scattered from theguided beam was observed and photographed. The intrinsic loss of theguide was estimated to be about 7 dB/cm. We expect that by perfection ofour technique this loss can be greatly reduced.

We have also demonstrated the filling of grooves by photopolymerizationof a liquid monomer film. A thin film of cyclohexyl methacrylate,containing about 2 percent benzoin methyl ether as a photosensitiveinitiator, was applied to an embossed PMMA substrate. Excess liquid wasallowed to evaporate for about forty minutes; and then the slide wasexposed for about ten minutes to the light of a one kilowatt I-Ig-Xe arclamp source with f/l .5 quartz optics and a water filter. Duringevaporation and exposure, the slide was maintained in a dry nitorgenatmosphere to eliminate oxygen, which would inhibit polymerization. Theguide 27 of FIG. 3C was formed. Light of 0.633 micrometer wavelength wascoupled into the guide 27 by a prism-film coupler, and the guidingaction was confirmed, as was done for the guide made by solutiondeposition. The poly(cyclohexyl methacrylate) film 26 has a refractiveindex of 1.505, exceeding the index of the PMMA substrate (n 1.490) byabout one percent.

It should be noted that the dies and guiding structures of FIGS. 2A-2Cand 3A-3C are very simple for purposes of illustration only. The die andthe resulting guiding structures can have any desired degree ofcomplexity.

Moreover, to speed mass production of some circuits, side-by-siclereplicas of the same circuit, in columns and rows, can be built into thesame die, as illustrated inthe modified flow diagram of FIG. 4.

In FIG. 4, the first three steps outline the making of a master die. Inthe first step, like the first step of FIG. 1, a first die is made witha surface pattern at least in part matching the desired guidinginterfaces for a single optical circuit. Next, a mold, which can besimilar to substrates 14 of FIGS. 28 and 3B, is embossed in laterallyoffset positions in columns and rows by repeated impressions by thatfirst die.

Third, the master die of a suitably hard material is made in the mold.The material of the master die could be, for example, any one of severalthermo-setting resins; or it could be a metal, such as nickel orchromium. The master die is then used to emboss the ultimatesubstrate'14 in which the ducplicate circuits are subsequently made bycoating with material 16 or 26. The completed assembly may then be cutapart in a regular pattern to separate the individual circuits, ifdesired. The master die may be used repeatedly.

Of course, the method of FIG. 4 could be modified to emboss the mold forthe master die with many different, interconnected circuit patterns,some of which may be repeated. This alternative is desirable for largescale circuit integration. We claim: l. A method of making dielectricwaveguiding devices, comprising the steps of making a die having asurface shape at least a portion of which matches that of a guidingdielectric interface in the desired waveguiding devices,

embossing the surface of a substantially optically transparent plasticsubstrate of a first index of refraction with said die to produce insaid substrate surface an impression in the pattern of said interface,

forming said interface by coating said substrate surface with a liquidmaterial capable of flowing to fill the impression in said surface andcapable of solidification into an optically transparent film in responseto applied energy, and

applying energy to said material to cause solidification of saidmaterial into an optically transparent dielectric film of a second indexof refraction higher than said first index, said film forming saidinterface with said substrate surface and having a thickness over saidimpression capable of guiding a beam of optical radiation.

2. A method according to claim l in which the coating step comprisesdepositing on the substrate an optically transparent solid material in aliquid solution, and in which the energy supplying step comprisessupplying heat to the solution to evaporate the solvent therefrom toleave said optically transparent dielectric film in the impression ofsaid substrate surface.

3. A method according to claim 1 in which the coating step comprisesdepositing on the substrate a film of a liquid monomer, and in which theenergy-applying step comprises initiating polymerization of said film toform said optically transparent dielectric film.

4. A method according to claim 1 in which the coating step comprisesdepositing on the substrate a film of a liquid monomer and,

evaporating a portion of the liquid until the desired thickness of saidfilm is produced, and in which the energy applying step comprisesinitiating polymerization of said film to form said opticallytransparent dielectric film.

5. A method according to claim 1 in which the coating step comprisesdepositing on the substrate a film of cyclohexyl methacrylate containingbenzoic methyl ether as a photosensitive initiator and,

evaporating a portion of the liquid until the desired thickness of saidfilm is produced, and in which said energy applying step comprisesexposing the film in a dry nitogen atmosphere with an ultraviolet lampto form a polymerized film as said optically transparent dielectricfilm.

6. A method according to claim 1 in which the coating step comprisessupplying the liquid material to the substrate surface as a solution,said solution having a viscosity and surface tension selected forforming a relatively smooth exterior surface as compared to the embossedsurface, and the energy applying step comprises supplying heat to thesolution by maintaining the coated substrate at a temperature thatcauses evaporation of the solvent.

7. A method according to claim 1 in which the coating step comprisessupplying the liquid material to the substrate surface as a liquidmonomer including a polymerization initiator, said liquid monomer havinga viscosity and surface tension selected for forming a relatively smoothexterior surface as compared to the embossed surface, and the energyapplying step comprises applying to said monomerenergy of the type towhich the initiator responds by initiating polymerization.

2. A method according to claim 1 in which the coating step comprisesdepositing on the substrate an optically transparent solid material in aliquid solution, and in which the energy supplying step comprisessupplying heat to the solution to evaporate the solvent therefrom toleave said optically transparent dielectric film in the impression ofsaid substrate surface.
 3. A method according to claim 1 in which thecoating step comprises depositing on the substrate a film of a liquidmonomer, and in which the energy-applying step comprises initiatingpolymerization of said film to form said optically transparentdielectric film.
 4. A method according to claim 1 in which the coatingstep comprises depositing on the substrate a film of a liquid monomerand, evaporating a portion of the liquid until the desired thickness ofsaid film is produced, and in which the energy applying step comprisesinitiating polymerization of said film to form said opticallytransparent dielectric film.
 5. A method according to claim 1 in whichthe coating step comprises depositing on the substrate a film ofcyclohexyl methacrylate containing benzoic methyl ether as aphotosensitive initiator and, evaporating a portion of the liquid untilthe desired thickness of said film is produced, and in which said energyapplying step comprises exposing the film in a dry nitrogen atmospherewith an ultraviolet lamp to form a polymerized film as said opticallytransparent dielectric film.
 6. A method according to claim 1 in whichthe coating step comprises supplying the liquid material to thesubstrate surface as a solution, said solution having a viscosity andsurface tension selected for forming a relatively smooth exteriorsurface as compared to the embossed surface, and the energy applyingstep comprises supplying heat to the solution by maintaining the coatedsubstrate at a temperature that causes evaporation of the solvent.
 7. Amethod according to claim 1 in which the coating step comprisessupplying the liquid material to the substrate surface as a liquidmonomer including a polymerization initiator, said liquid monomer havinga viscosity and surface tension selected for forming a relatively smoothexterior surface as compared to the embossed surface, and the energyappLying step comprises applying to said monomer energy of the type towhich the initiator responds by initiating polymerization.